1. Field of the Invention
The present invention relates to a semiconductor laser device for controlling changes in the emission wavelength of laser light emitted out of a semiconductor laser by means of an optical fiber grating. The present invention also relates to an optical fiber amplifier which uses the semiconductor laser device.
2. Description of the Prior Art
FIG. 11 is a diagram showing the structure of a prior art semiconductor laser device. In FIG. 11, reference numeral 110 denotes a pump laser module that emits laser light, reference numeral 120 denotes an optical fiber for guiding the laser light from the pump laser module 110, and reference numeral 130 denotes an optical fiber grating formed in the optical fiber 120.
Furthermore, in the pump laser module 110 of FIG. 11, reference numeral 111 denotes a 980-nm band semiconductor laser (i.e. laser diode), reference numeral 112 denotes a temperature monitor for monitoring the temperature of the pump laser module 110, reference numeral 113 denotes a cooler for keeping the temperature of the pump laser module 110 constant according to the monitoring result of the temperature monitor 112, and reference numeral 115 denotes a coupling optical system for coupling light emitted out of the semiconductor laser 111 into an optical fiber 120.
980-nm band laser light is used for the excitation of an erbium-doped fiber amplifier (EDFA). Since the gain-wavelength characteristic of EDFA changes when the emission wavelength of the laser light changes during the excitation, an optical fiber grating 130 is disposed at the output of the pump laser module 110 as measures against changes in the gain-wavelength characteristic.
FIG. 12 is a diagram showing an example of the structure of the semiconductor laser 111. In FIG. 12, reference numeral 111a denotes an n-type electrode, reference numeral 111b denotes a GaAs substrate, reference numeral 111c denotes an n-type cladding layer, reference numeral 111d denotes a multiple quantum well (MQW) active layer, reference numeral 111e denotes a p-type cladding layer, and reference numeral 111f denotes a p-type electrode. In the prior art semiconductor laser device, the semiconductor laser 111 having the MQW active layer 111d is used.
FIG. 13 is a diagram showing an energy band structure in the vicinity of the MQW active layer 11d of the semiconductor laser 111. In FIG. 13, reference numeral 142 denotes a conduction band, reference numeral 143 denotes a valence band, reference numerals 146A and 146B denote quantum wells, respectively, reference numeral 147 denotes a barrier layer, reference numeral 144 denotes a guide layer, and reference numeral 145 denotes a cladding layer. Each of the two quantum wells 146A and 146B is composed of InGaAs of In chemical composition of 0.2. The barrier layer 147 is composed of AlGaAs of Al chemical composition of 0.2. The guide layer 144 is composed of AlGaAs of Al chemical composition of 0.2. The cladding layer 145 is composed of AlGaAs of Al chemical composition of 0.48.
In general, the number of wells included in the MQW active layer 111d ranges from 2 to 4. Each of the two quantum wells 146A and 146B has a thickness Lz ranging from 5 nm to 15 nm, the barrier layer 147 has a thickness Lb ranging from 10 nm to 50 nm, and the guide layer 144 has a thickness ranging from 10 nm to 500 nm. The Al chemical composition of the above-mentioned AlGaAs is adjusted between 0.0 and 0.5 from the viewpoint of optical confinement.
Population inversion is formed by an electric current""s flowing in a forward direction between the p-type electrode 111f and the n-type electrode 111a, and hence injecting electrons and holes into the MQW active layer 111d. As a result, the semiconductor laser 111 oscillates at a 980-nm band of emission wavelengths determined by the bandgap of the MQW active layer 111d, and emits laser light to the optical fiber 120 by way of the coupling optical system 115.
In general, since the semiconductor laser uses interband transitions, it has a gain over a wide wavelength range (e.g., ten-odd nm). The emission wavelength of the semiconductor laser 111 differs and changes according to chip-to-chip variations and change in temperature. Therefore, the change in the emission wavelength of the semiconductor laser device is controlled by the optical fiber grating 130 disposed as an external resonator in the prior art semiconductor laser device. For example, details of the semiconductor laser device provided with the optical fiber grating 130 are disclosed in  less than Reference 1 greater than .
 less than Reference 1 greater than : Martin Achtenhagen, et al.: xe2x80x9cL-I Characteristics of Fiber Bragg Grating Stabilized 980-nm Pump Lasersxe2x80x9d, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 5, MAY 2001.
When the temperature of the pump laser module 110 changes greatly because of a self heating of the semiconductor laser 111 and change in ambient temperature, the wavelength characteristic of the threshold gain distribution also changes. On the other hand, since the wavelength characteristic of the optical fiber grating 130 remains fixed, the semiconductor laser 111 does not oscillate in external resonance mode and therefore the emission wavelength cannot be kept constant.
To avoid this problem, a temperature control mechanism is disposed in the semiconductor laser device of FIG. 11. In other words, the prior art semiconductor laser device is so constructed as to monitor the temperature of the pump laser module 110 by means of the temperature monitor 112, to control an electric current flowing through the cooler 113 by means of a temperature controller not shown in the figure, and to keep the temperature of the pump laser module 110 constant. Thus, the semiconductor laser device can stabilize the emission wavelength, and can control the change in the gain-wavelength characteristic when applied to EDFA. Japanese patent application publication No. 2000-353856 discloses a prior art technology associated with the semiconductor laser device mentioned above, for example.
A problem with a prior art semiconductor laser device constructed as mentioned above is that to keep the emission wavelength constant the semiconductor laser device has to have a temperature control mechanism that consists of a temperature monitor, a temperature controller, a cooler, etc., and the structure of the semiconductor laser device therefore becomes complex.
The present invention is proposed to solve the above-mentioned problem, and it is therefore an object of the present invention to provide a semiconductor laser device having a simple structure and capable of keeping the emission wavelength constant without having to use a temperature control mechanism.
It is another object of the present invention to provide a semiconductor laser device capable of controlling the change in the emission wavelength by means of a temperature control mechanism with low control resolution or low control performance.
It is a further object of the present invention to provide an optical fiber amplifier provided with such a semiconductor laser device as a source of pumping light, and capable of controlling the change in the gain-wavelength characteristic.
In accordance with an aspect of the present invention, there is provided a semiconductor laser device, comprising: an optical fiber having an optical fiber grating; a semiconductor laser having an active layer with a single quantum well, for emitting laser light; and a coupling optical system for coupling the laser light emitted out of the semiconductor laser into the optical fiber.
In accordance with another aspect of the present invention, the coupling optical system includes a narrow-band filter for adjusting an incident angle of the laser light emitted out of the semiconductor laser.
In accordance with a further aspect of the present invention, the optical fiber grating has a reflection bandwidth wider than or substantially equal to a 3 dB bandwidth of a gain of the semiconductor laser or a spectrum full width at half maximum of the laser light of the semiconductor laser.
In accordance with another aspect of the present invention, the coupling optical system has a narrow-band filter for adjusting an incident angle of the laser light emitted out of the semiconductor laser. Furthermore, the optical fiber grating has a reflection bandwidth wider than or substantially equal to a 3 dB bandwidth of a gain of the semiconductor laser or a spectrum full width at half maximum of the laser light of the semiconductor laser.
In accordance with a further aspect of the present invention, the coupling optical system includes a collimator lens for collimating the laser light emitted out of the semiconductor laser and for outputting the collimated laser light to the narrow-band filter, and a condenser lens for focusing the laser light output from the narrow-band filter onto the optical fiber.
In accordance with another aspect of the present invention, the semiconductor laser has an anti-reflection coating with a reflectivity of about 10% or less, which is formed on an emitting exit face thereof from which the laser light is emitted.
In accordance with a further aspect of the present invention, the anti-reflection coating has a reflectivity lower than that of the optical fiber grating.
In accordance with another aspect of the present invention, the semiconductor laser includes a layer having a refraction index lower than that of an optical guide layer disposed outside the active layer with the single quantum well, the layer having such a thickness as to prevent itself from becoming a barrier that keeps an electric current from flowing through the semiconductor laser and the layer being disposed outside the optical guide layer.
In accordance with a further aspect of the present invention, the active layer, a barrier layer, and a guide layer of the semiconductor laser are configured to have a distortion compensating structure.
In accordance with another aspect of the present invention, the optical fiber grating has a reflection bandwidth of 5 nm or more.
In accordance with a further aspect of the present invention, the narrow-band filter includes an incident angle adjusting mechanism for adjusting the narrow-band filter so that the incident angle of the laser light incident on the narrow-band filter approaches 90 degrees with increasing ambient temperature.
In accordance with another aspect of the present invention, the active layer with the single quantum well of the semiconductor laser has a thickness ranging from 10 nm to 25 nm.
In accordance with a further aspect of the present invention, there is provided a semiconductor laser device, comprising: an optical fiber having an optical fiber grating; a semiconductor laser having an active layer with two or more quantum wells formed at intervals that are close enough to provide quantum coupling, for emitting laser light; and a coupling optical system for coupling the laser light emitted out of the semiconductor laser into the optical fiber.
In accordance with another aspect of the present invention, the coupling optical system includes a narrow-band filter for adjusting an incident angle of the laser light emitted out of the semiconductor laser.
In accordance with a further aspect of the present invention, the optical fiber grating has a reflection bandwidth wider than or substantially equal to a 3 dB bandwidth of a gain of the semiconductor laser or a spectrum full width at half maximum of the laser light of the semiconductor laser.
In accordance with another aspect of the present invention, the coupling optical system has a narrow-band filter for adjusting an incident angle of the laser light emitted out of the semiconductor laser. Furthermore, the optical fiber grating has a reflection bandwidth wider than or substantially equal to a 3 dB bandwidth of a gain of the semiconductor laser or a spectrum full width at half maximum of the laser light of the semiconductor laser.
In accordance with a further aspect of the present invention, the coupling optical system includes a collimator lens for collimating the laser light emitted out of the semiconductor laser and for outputting the collimated laser light to the narrow-band filter, and a condenser lens for focusing the laser light output from the narrow-band filter onto the optical fiber.
In accordance with another aspect of the present invention, the semiconductor laser has an anti-reflection coating with a reflectivity of about 10% or less, which is formed on an emitting exit face thereof from which the laser light is emitted.
In accordance with a further aspect of the present invention, the anti-reflection coating has a reflectivity lower than that of the optical fiber grating.
In accordance with another aspect of the present invention, the active layer, a barrier layer, and a guide layer of the semiconductor laser are configured to have a distortion compensating structure.
In accordance with a further aspect of the present invention, the two or more quantum wells are formed at intervals of 8 nm or less.
In accordance with another aspect of the present invention, the optical fiber grating has a reflection bandwidth of 5 nm or more.
In accordance with a further aspect of the present invention, the narrow-band filter includes an incident angle adjusting mechanism for adjusting the narrow-band filter so that the incident angle of the laser light incident on the narrow-band filter approaches 90 degrees with increasing ambient temperature.
In accordance with another aspect of the present invention, there is provided an optical fiber amplifier comprising: a semiconductor laser device including an optical fiber having an optical fiber grating, a semiconductor laser having an active layer with a single quantum well, for emitting pumping light, and a coupling optical system for coupling the pumping light emitted out of the semiconductor laser into the optical fiber; a pumping light-signal light coupling unit for coupling the pumping light emitted out of the semiconductor laser device to signal light; and a rare-earth-doped optical fiber that is pumped by the pumping light so as to amplify the signal light output from the pumping light-signal light coupling unit.
In accordance with a further aspect of the present invention, there is provided an optical fiber amplifier comprising: a semiconductor laser device including an optical fiber having an optical fiber grating, a semiconductor laser having an active layer with two or more quantum wells formed at intervals that are close enough to provide quantum coupling, for emitting pumping light, and a coupling optical system for coupling the pumping light emitted out of the semiconductor laser into the optical fiber; a pumping light-signal light coupling unit for coupling the pumping light emitted out of the semiconductor laser device to signal light; and a rare-earth-doped optical fiber that is pumped by the pumping light so as to amplify the signal light output from the pumping light-signal light coupling unit.
Accordingly, in accordance with the present invention, the semiconductor laser device can keep the emission wavelength of the laser light constant with a simple structure and without any temperature control mechanism. In addition, by using a temperature control mechanism which is so simply structured that its temperature control resolution or its temperature control performance is reduced, the semiconductor laser device can control changes in the emission wavelength.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.